Winter Moon Dance

Jupiter is high in the sky throughout most of this December’s evenings, bright and easy to observe.

As I wrote last month, this year’s Jupiter opposition is an unusually large, close one, and it’s even reasonably high in the sky, transiting at 63 degrees, You should be able to see all its major bands, the four Galilean satellites and their shadows, and the great red spot (GRS) with nearly any telescope. On a good night with good optics, crank up the magnification and you can see a lot more. Look for festoons – white swirls of turbulence, especially in the area following the GRS– and barges, unexplained small dark spots.

Double shadow transits – times when two or more of Jupiter’s biggest moons are casting their shadows on the planet – are among the easiest and most fun Jovian shows to watch. Unfortunately, they’ve been rare of late. The only one I could find this month is on Tuesday night, December 27, but it’s an exceptionally good show. The fun starts before it’s fully dark, a bit at 5:30, when Europa begins a transit of Jupiter’s face. The three other moons are all clustered nearby.

About ten minutes later, Ganymede’s shadow begins a transit right next to Europa. Transits of the moons themselves are often hard to follow: it’s easy to see a moon standing out against the relatively dark limb of the planet, but then you tend to lose track of the moon as it moves in toward the brighter center of Jupiter’s disk. In this case, you’ll be able to follow Ganymede’s shadow all the way across the disk to help you find Europa.

Shortly after 7:35, Europa’s shadow enters the disk’s southeast limb, while Europa and Ganymede’s shadow continue their allemande over on the southwest edge. Europa and the shadow finally exit the disk around 8 p.m. (Europa first, then Ganymede’s shadow). Europa’s shadow continues its solo transit for another two hours, until a bit after 10 p.m.

It all looks like an excellent show, of a sort that has become all too rare lately, so check it out if your skies are clear that day. Maybe you can even share a view with friends and relatives visiting over the holidays – or help someone with that Christmas scope they got at the SJAA swap meet last month.

If you get tired of Jupiter (horrors!), you can still catch Uranus and Neptune in December skies. But start early in the evening, as soon as it’s fully dark: they begin the evening fairly low in the south and get even lower as the evening progresses. Uranus is Pisces, above Pegasus’ withers (at least if you turn your head upside down so that Pegasus isn’t lying on his back). Neptune is in Aquarius, near the Capricornus border and about six degrees off Capricornus’ left horn.

Venus, too, is in the early evening sky, in gibbous phase; it’s relatively close to the sun, so catch it shortly after sunset.

Mercury moves into the dawn sky during the latter half of December, joining Saturn and Mars. Pluto is too close to the sun to observe this month.

December 10th brings us a total lunar eclipse. It is visible in San Jose, but there’s a catch: totality lasts from about 6 to 7 am. So those willing to rise early on a Saturday morning will catch a lovely show. Or if you’re up late observing on that full-moon Friday night, you can catch the beginning of the eclipse: the penumbral eclipse starts at about 3:30 am, though it takes a sharp eye to spot any difference during the penumbral phase. The partial (umbral) eclipse starts at 4:45 a.m. Visit eclipse.gsfc.nasa.gov for more detailed timings.